Solid state transformer controller

Abstract

A decoupled system for controlling a solid state transformer (SST), the SST comprising an AC-to-DC stage, a DC-to-AC stage, and a DC-to-DC stage, the DC-to-DC stage comprising one or more DC-to-DC converters. The system comprises a stored energy controller coupled to the AC-to-DC stage, the energy controller configured to control the total amount of stored energy within the capacitors of the SST; a power flow controller coupled to the DC-to-AC stage, the power flow controller configured to control power flow in the SST; and one or more energy balancing controllers each coupled to a corresponding DC-to-DC converter, each energy balancing controller configured to balance energy in the corresponding DC-to-DC converter.

The invention claimed is: 
     
       1. A system for controlling a solid state transformer (SST), the SST comprising a high-voltage-side AC-to-DC stage, a low-voltage-side DC-to-AC stage, a DC-to-DC stage coupled between the HV-side AC-to-DC stage and the LV-side DC-to-AC stage, the DC-to-DC stage comprising one or more DC-to-DC converters, and a set of capacitors for storing energy therein, the system comprising:
 a stored energy controller coupled to the HV-side AC-to-DC stage, the stored energy controller configured to control a total amount of the stored energy within the capacitors of the SST; 
 a power flow controller coupled to the LV-side DC-to-AC stage, the power flow controller configured to control power flow in the SST; and 
 one or more energy balancing controllers, each energy balancing controller of the one or more energy balancing controllers coupled to a corresponding DC-to-DC converter of the one or more DC-to-DC converters, each energy balancing controller configured to balance energy in the corresponding DC-to-DC converter, 
 wherein the stored energy controller, the power flow controller and the one or more energy balancing controllers are decoupled from one another such that the stored energy controller is configured to operate independently of the power flow controller and the one or more energy balancing controllers, such that the power flow controller is configured to operate independently of the stored energy controller and the one or more energy balancing controllers, and such that each of the one or more energy balancing controllers is configured to operate independently of the stored energy controller and the power flow controller. 
 
     
     
       2. The system of  claim 1 , wherein the stored energy controller, the power flow controller and the one or more energy balancing controllers are decoupled at a function level. 
     
     
       3. The system of  claim 1 , wherein the stored energy controller, the power flow controller and the one or more energy balancing controllers are decoupled at a state variable control level. 
     
     
       4. The system of  claim 1 , wherein the stored energy controller, the power flow controller and the one or more energy balancing controllers are configured with independent control objectives. 
     
     
       5. The system of  claim 1 , wherein the stored energy controller controls the HV-side AC-to-DC stage, the HV-side AC-to-DC stage comprising a plurality of HV-side AC-to-DC converters, each HV-side AC-to-DC converter of the plurality of HV-side AC-to-DC converters operative for charging/discharging a respective high voltage (HV) capacitor, to regulate the total amount of energy stored in the capacitors of the SST according to: 
       
         
           
             
               
                 dE 
                 dt 
               
               = 
               
                 
                   P 
                   HV 
                 
                 - 
                 
                   P 
                   LV 
                 
               
             
           
         
         where E=0.5 (Σ j=1   N  Cj HV vj HVdc   2 +C LV v LVdc   2 ), vj HVdc  is a voltage of j th  HV-side capacitor, Cj HV  is a capacitance of j th  capacitor in HV-side, v LVdc  is a voltage of LV-side capacitor, C LV  is a capacitance of the LV-side capacitor, P HV =Σ j=1  Pi HV  with P HV  being the active power passing through HV-side AC-to-DC converter, and P LV  is the active power passing through the LV-side DC-to-AC converter. 
       
     
     
       6. The system of  claim 5 , wherein the power flow controller controls the LV-side DC-to-AC stage, the LV-side DC-to-AC stage comprising a LV-side DC-to-AC converter that charges/discharges the LV-side capacitor to satisfy P LV . 
     
     
       7. The system of  claim 6 , wherein the one or more energy balance controllers is configured to indirectly control a dynamic HV-side capacitor voltage based on a state variable defined by: 
       
         
           
             
               
                 
                   
                     d 
                     ⁢ 
                     Δ 
                     ⁢ 
                     Ej 
                   
                   dt 
                 
                 = 
                 
                   Pj 
                   
                     dc 
                     - 
                     dc 
                   
                 
               
               , 
               
                 ( 
                 
                   
                     j 
                     = 
                     1 
                   
                   , 
                   2 
                   , 
                   … 
                       
                   , 
                   N 
                 
                 ) 
               
               , 
             
           
         
         where ΔEj=0.5 (Cj HV vj HVdc   2 −C LV v LVdc   2 ), (j=1, 2, . . . , N), C LV  is the capacitance of the LV-side capacitor, and v LVdc  is the voltage of LV-side capacitor. 
       
     
     
       8. The system of  claim 5 , wherein the one or more energy balance controllers is configured to indirectly control a respective dynamic capacitor voltage to actively remove or regulate the magnitude of voltage ripple on the HV-side capacitor voltage. 
     
     
       9. The system of  claim 1 , wherein each controller is a proportional integral controller. 
     
     
       10. The system of  claim 1 , wherein power reference for each DC-to-DC converter is generated by adding a feedforward compensation to proportional integral output. 
     
     
       11. The system of  claim 1 , wherein power in the one or more DC-to-DC converters is regulated by a phase shift switching strategy. 
     
     
       12. A method for controlling a solid state transformer (SST) using a system according to  claim 1 , comprising:
 controlling stored energy in the SST using the stored energy controller; 
 controlling power flow in the SST using the power flow controller; and 
 balancing energy in the corresponding DC-to-DC converter using the one or more energy balancing controllers. 
 
     
     
       13. The method of  claim 12 , wherein the controlling stored energy occurs at a first location, the controlling power flow occurs at a second location and the balancing energy occurs at a third location, wherein at least two of the first location, second location and third location are spaced from each other.